The semiconductor light-emitting diode is a solid state device that emits light when a voltage is applied across it. In conventional inorganic semiconductor devices, a p-n junction is fabricated, consisting of a single crystal semiconductor formed with one part of the crystal doped with positive charged carriers (designated n-type) and another part of the crystal doped with negative charged carriers (designated n-type). It is a basic feature of all such p-n junctions that their chemical composition and, hence, doping profile is static, or fixed in position in the host crystal. During operation, charge carriers are injected into or removed from the junction through electrical contacts external to the junction region.
Junction regions are introduced in a variety of manners. Abrupt junctions, in which the transition between the n- and p-type regions are relatively narrow, are typically fabricated by alloying a solid impurity (for example, a metal) with the semiconductor, or by employing one of a number of epitaxial growth techniques on crystalline semiconductor substrates. Graded junctions, in which the transition region is relatively broader, are produced by diffusion or ion implantation of impurities into the host semiconductor. These technologically demanding manufacturing processes make it both difficult and expensive to fabricate large area displays. Moreover, such devices are inherently brittle and lack the mechanical and processing advantages generally associated with organic, and especially polymeric, materials. For these reasons, there has been considerable interest for many years in organic materials for use as the active (light-emitting) components of light-emitting diodes.
A number of electroluminescent devices have been disclosed which use organic materials as an active light-emitting layer in a sandwich architecture. For example, S. A. Van Slyke and C. W. Tang in U.S. Pat. No. 4,539,507 disclosed a device having a bilayer of two vacuum-sublimed films of small organic molecules sandwiched between two contacts. R. H. Friend et al. in U.S. Pat. No. 5,247,190 disclosed a device having a thin dense polymer film made up of at least one conjugated polymer sandwiched between two contacts. Because these are electric field-driven devices, the active electroluminescent layer must be very thin (typically about 1000 angstroms thick) and uniform. In these devices, excess charge carriers are injected through the contacts into the light-emitting semiconductor layer by processes well known in the study of metal-semiconductor interfaces (see, e.g., M. A. Lampert and P. Mark, Current Injection in Solids, Academic Press, NY, 1970). Dissimilar metals were used for the contacts to facilitate the injection of electrons at one contact and the injection of holes at the other. As a result, the current-voltage characteristic curves of these devices show a pronounced asymmetry with respect to the polarity of the applied voltage, like that typical of the response of diodes. Hence, the rectification ratio of such devices is high, typically greater than 10.sup.3, and light is emitted for only one polarity of the applied voltage.
Among other drawbacks, the devices disclosed by S. A. Van Slyke and C. W. Tang and by R. H. Friend et al. suffer from the need to use metals of relatively low work function to inject sufficient numbers of electrons into the active layers to produce efficient light output at low drive voltages. Because such metals are readily oxidized, they are a source of device degradation in ambient conditions and require passivating packaging. To improve efficiency and operational stability, low work function metals such as Mg and Li were alloyed with more stable metals with higher work function such as Al or Ag and used as the cathode for organic LEDs (for example C. W. Tang and S. A. Van Slyke, U.S. Pat. No. 4,885,211 (1989); S. A. Van Slyke and C. W. Tang, U.S. Pat. No. 5,059,862 (1991)). The efficiencies were higher tand when the higher work function metal was used alone. However, when the proportion of a higher work function metal was increased beyond a certain level, the initial and extended performance were significantly reduced. A double-layer cathode was also proposed where a high work function metal was overcoated on the top of the low work function metal (for example, J. Kido, K. Nagai and Y. Okamoto, IEEE Trans. Electron Devices, 40 (1993), 1342).
The class of materials suitable for organic LEDs was expanded by Burroughes et al (J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. MacKay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature, 347 (1990), 539) who demonstrated that LEDs could be made from conjugated polymers such as poly(phenylenevinylene), PPV. Devices using aluminum as a cathode disclosed by Burroughes et al had low brightness and low efficiency (0.002%) (D. D. C. Bradley, Synth. Met., 54 (1993), 401) and operated only at very high voltages (40 volts) (R. H. Friend, J. H. Burroughes and D. D. Bradley, U.S. Pat. No. 5,247,190 (1993)). Braun et al (D. Braun and A. J. Heeger, Appl. Phys. Lett., 58 (1991), 1982) showed that by using the soluble PPV derivative, poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene), MEH-PPV with a low work function electron injecting cathode, such as calcium, polymer LEDs could achieve high brightness with external quantum efficiencies of order 1% (U.S. Pat. No. 5,408,109).
For a given polymer, a common practice is to use conduction band and valence band offsets at semiconductor heterojunction interfaces to achieve carrier confinement within the active electroluminescent (EL) layer. Electron-transport layers, which contain either small molecules or an electrically active polymer, are placed between the luminescent layer and cathode in order to enhance the quantum efficiency through charge carrier confinement (N. C. Greenham, S. C. Morrati, D. D. C. Bradley, R. H. Friend and A. B. Holmes, Nature, 365 (1993), 628; A. R. Brown, D. D. C. Bradley, J. H. Burroughes, R. H. Friend, N. C. Greenham, P. L. Holmes and A. Kraft, Appl. Phys. Lett., 61 (1990), 2793; S. Aratani, C. Zhang, K. Pakabaz, S. Hoger, F. Wudl and A. J. Heeger, J. Electron. Mater., 22 (1993), 745). The quantum efficiencies are also improved, when the electron-transporting molecules are blended with the luminescent polymer (C. Zhang, S. Hoger, K. Pakbaz, F. Wudl and A. J. Heeger, J. Electron. Mater., 23 (1994), 453; Z. Hu, Y. Yang and F. E. Karasz, J. Appl. Phys., 76 (1994), 2419I; D. Parker and Q. Pei, Appl. Phys. Lett., 65 (1994), 1272; I. N. Kang, D. H. Hwang, H. K. Shim, T. Z. Zyung, and J. J. Kim, Macromolecules, 29 (1996), 165). Although these approaches are promising, the device performance remains well below that obtained with devices which use a low work function metal as cathode; the latter providing significantly higher efficiency and higher brightness at low operating voltages.
Q. Pei et al discovered the polymer light-emitting electrochemical cell EEC) (Q. Pei, G. Yu, C. Zhang, Y. Yang, and A. J. Heeger, Science 269 (1995), 1086; Q. Pei and F. Klavetter, U.S. patent Ser. No. 08/268,763 (Jun. 28, 1994)) which contains solid electrolyte in the electroluminescent layer. The ionic conducting character of this device allows the creation of a p-n junction in situ by electrochemical doping. Because carrier injection occurs through ohmic contacts into the doped n-type and p-type regions, stable metals such as Al and Au can be used for the anode and cathode. Thus, the quantum efficiency in LECs is relatively independent of the work function of the cathode and anode since there is no need to match the work function of the electrodes to the .pi. and .pi.* energies of a luminescent polymer. However, since the in situ creation of the p-n junction is limited by ionic transport (the ionic conductivity of the active layer), the time required for the device to turn-on is considerably longer than in conventional LEDs, where all processes are electronic.